1. Field of the Invention
This invention relates to a process for producing nanoparticles and more particularly to a process for reacting a metal reactant and an oxidizing agent to make nanoparticles, the process being capable of eliminating coarse tail and scale formation.
2. Description of the Related Art
The chloride process for making titanium dioxide includes high-temperature anhydrous vapor phase reactions where liquid titanium tetrachloride is vaporized and superheated then reacted with oxygen to produce titanium dioxide. The superheating and subsequent reaction phase can be carried out either by a refractory process, where the reactants are heated by refractory heat exchangers and combined. Alternatively, carbon monoxide can be purified and then mixed with the titanium tetrachloride and oxidizing agent and then the mixture subjected to a controlled combustion. Another method is by vaporizing the titanium tetrachloride in a hot plasma along with the oxidizing agent.
The development of these processes for the production of fine particles which are below about 100 nm in size, termed “nanoparticles”, has been a point of focus in recent years. In particular, titanium dioxide nanoparticles have gained increased attention because they can have a high degree of transparency and they can also have UV protective properties. The combined properties of transparency and UV protection is especially desirable in applications demanding both properties including, without limit, cosmetics; product coatings, such as automotive clear coatings and wood coatings; and plastics, such as polymer composites.
The development of processes for making nanoparticles continues to be a challenge.
The build-up of scale within the reactor is a significant problem in the production of metal oxide nanoparticles, particularly titanium dioxide nanoparticles. Scale is a layer of solids formed on the walls of the reactor that can build up significantly overtime as the hot metal oxide particles and reactants collide with the walls of the reactor and stick at a temperature at which the metal oxide particles can coalesce. The layer can comprise sintered metal oxides which are very hard and tenacious. This hard and tenacious type of reactor wall scale is labor intensive to remove and represents loss of product which increases production costs.
In the production of titanium dioxide nanoparticles the presence of “coarse tail” can be a significant problem. “Coarse tail” is an amount of large particles, typically having a diameter exceeding about 100 nm and greater, present in the product. The large particles can be built up from smaller metal oxide particles and/or reactants which collide with each other and coalesce at a high temperatures. In addition, the large particles can result from particle aggregates that can form from partially coalesced particles. Further, a “soft” layer of large coalesced particles that can form on the walls of the reactor can become entrained with the flow of product and contribute to coarse tail.
In the manufacture of titanium dioxide nanoparticles, coarse tail can be a commercialization barrier because it is considered detrimental to transparency. Even a very small percentage of titanium dioxide particles having a diameter above about 100 nm can impart a degree of opacity sufficient to render the product unacceptable for high transparency applications such as automotive clear coatings. Since, large particles can be difficult and costly to remove there is a need for processes capable of producing nanoparticles which are free of coarse tail.
In U.S. Pat. No. 6,277,354 at Col. 4, lines 37-41 this “stickiness” property of metal chlorides and metal oxides which can lead to wall scale and coarse tail is defined as meaning that the ratio of the temperature Kelvin of the particular particles to their melting point temperature Kelvin is equal to or less than about ⅔.
A highly turbulent quench zone has been described for controlling particle size distribution and reactivity to overcome particle growth and agglomeration. Highly turbulent quenching conditions can also provide high conversion rates. While relatively high conversions of reactants can be an advantage of this process, coarse tail and reactor wall scale problems remain. Highly turbulent conditions promote collisions between particles which at high temperatures increase particle coalescence which increases the proportion of large particles and the buildup of reactor wall scale.